The effective radio-frequency transmission power of mobile radio stations is generally matched to various parameters relating to the transmission systems, for example to the distance between the mobile station and the base station, to the radio channel attenuation, to the noise on the radio channel, to the spectral characteristics of the radio channel, and to the transmission method, etc. There are various analog or digital control signals in the mobile station in order to adjust the radio-frequency transmission power, and these control the gain and the operating point of the power amplifier. In order to keep the desired transmission power constant over a specific time period, for example over one time slot, these control signals are often supplied as a control variable to a control system which is in the form of a control loop. In particular for mobile radio systems whose modulation signal has an envelope curve with a constant amplitude (for example GSM with GMSK modulation (Gaussian Minimum Shift Keying)), solutions are known from the prior art in which the transmission power is slaved to a predetermined nominal value with the aid of a control loop. The control system is in this case designed such that it can compensate for fluctuations in the battery voltage or thermal effects on the power amplifier, so that a constant power level, which corresponds to the control variable, is transmitted.
One such solution from the prior art is implemented in the PMB 6850, E-GOLD+V1.2 product from the applicant, which is commercially available.
In order to transmit higher data rates, modulation methods whose envelope curve does not have a constant amplitude will also be used in the future. This is the situation, for example, in the EDGE mobile radio standard, in which 3π/8-8-PSK modulation is used. A similar, but narrowband method is likewise used in places in North America, namely TIA/EIA-136 with π/4-DQPSK modulation. During the active part of the data transmission, that is to say during the active transmission phases, amplitude fluctuations of up to 15 dB occur with these methods. Owing to these amplitude fluctuations of the modulation signal, it is impossible with these modulation methods to use gain control in the form of a control loop to stabilize the transmission power. A control system such as this would to a certain extent partially compensate for the amplitude modulation and would thus lead to incorrect transmission.
If the battery voltage or the current output level now decreases owing to a decrease in capacity within a time slot, or if the efficiency of the power amplifier decreases during a time slot owing to thermal loading, or if the operating point of the power amplifier is shifted, then this results in a noticeable fall in the transmitted power during the course of the time slot. This effect is particularly noticeable at high transmission power levels, at which the battery or the rechargeable battery for the mobile radio briefly has to supply a current of several amperes.
In the solutions from the prior art, in which a modulation method whose amplitude was not constant was used, the power drop was either tolerated or was partially reduced by means of appropriately overdesigned battery buffering. Solutions such as these are also included in the PMB 6850, E-GOLD+V1.2 product.